An attempt was made to develop a system for accurately and repeatably measuring the electrical properties of materials used in phantom modeling research. The interest was in the frequency range of 10 to 100 megahertz. The basis for the system as developed was the antenna modeling theorem as it relates the impedance of a short monopole antenna in air to its impedance in dielectric media with high radiofrequency power absorption, such as phantom modeling materials and biological tissues. For implementation it was necessary to provide a signal source, network analyzer, and small in- vivo probe. Comparisons were made of the measured electrical property data with corresponding reference data, as published in the open literature, to determine the accuracy and repeatability of the system as designed. This data comparison involved the following samples: water, methanol (67561), ethylene-glycol (107211), and saline solutions. Accuracies were within plus or minus 5% for both discrete and swept frequency measurements.